The present invention relates to blow molding in general, and more particularly to improvements in multi-stage methods of transforming blowable thermoplastic bubbles or parisons into bottles or other types of hollow shaped articles. Still more particularly, the invention relates to improvements in blow molding methods which involve conversion of an extruded or injection molded parison which is open at one end and closed at the other end into a pre-form and subsequent conversion of the pre-form into a finished hollow article, such as a bottle or the like. Methods of the just outlined character are practiced by resorting to a pre-form blow mold wherein a parison is converted into a pre-form whose volume exceeds the volume of the parison but is less than the volume of the finished article, and to a final blow mold wherein the pre-form is transformed into a finished article. It is further known to mechanically stretch the pre-form in the final blow mold and to effect pneumatic expansion of the pre-form in two stages the first of which includes converting the major (tubular) portion of the pre-form into the corresponding portion (tubular wall) of the shaped article and the second of which includes converting the remaining portion, namely, the closed end of the pre-form, into the corresponding portion (bottom) of the shaped article.
The method of the present invention belongs to that group of blow molding methods which involve biaxial orientation of thermoplastic material. The main difference between standard blow molding techniques and a method which involves biaxial orientation is that the latter method is carried out in several stages, i.e., pneumatic expansion of a parison into full contact with the surface bounding the cavity of a blow mold includes at least two steps. Thus, the parison is converted into a so-called pre-form whose volume exceeds the volume of the parison but is less than the volume of the shaped article, and the pre-form is thereupon pneumatically expanded (either in a single step or in several steps) to complete the transformation into a hollow shaped article. Multi-stage conversion of parisons into bottles or the like is intended to bring about pronounced stretching of molecules of the thermoplastic material which constitutes the tubular wall (i.e., the major portion) of the shaped article. To this end, the deformation takes place while the material which is to form the wall is maintained close to the lower limit of the acceptable temperature range and is thereupon caused or allowed to set by cooling (this is known as "freezing" of the material of the wall). Such mode of treatment enhances the physical characteristics (especially the strength) of the wall. It has been found that the strength of a wall which was subjected to biaxial orientation is much more pronounced than the strength of a wall which was formed in accordance with standard blow molding techniques. Greater strength of the wall material is desirable for several reasons, i.e., to enhance the stability of the corresponding major portion of the finished article or to allow for a reduction of the wall thickness of the article.
More specifically, the present invention relates to that class of blow molding methods which involve biaxial orientation of theremoplastic material and which further involve mechanical stretching of the pre-form, either prior to or simultaneously with conversion of the pre-form into a finished article. As a rule, mechanical stretching is effected by resorting to a blow tube or mandrel which includes a reciprocable core or rod that is movable against the inner side of the closed end of a pre-form and makes the pre-form longer by moving its closed end in a direction away from the open end. However, it is also known to grip the closed end of a pre-form from the outside and to pull the thus gripped closed end in a direction away from the open end. Reference may be had to U.S. Pat. No. 3,781,395, which discloses a blow molding apparatus wherein a parison is introduced into a pre-form blow mold and is converted into a bag-like pre-form. At the same time, the neck portion of the parison is converted into the neck portion of the finished shaped article by compression molding between the periphery of a blow tube or an insert which extends into the open end of the parison and the surrounding neck portion of the pre-form mold. The pre-form is thereupon introduced into a final blow mold wherein the neck portion is engaged by the adjacent part of the mold and by the blow tube or by a discrete insert. The blow tube or the insert carries a reciprocable rod-like stretching member which serves to move the closed end of the pre-form away from the neck portion. The stretching member may constitute a blow tube, i.e., it can have one or more passages and one or more orifices for admission of a blowing fluid into the interior of the pre-form. When the outer side of the closed end of the pre-form reaches or comes very close to the bottom surface of the final blow mold, the pre-form is expanded in response to admission of a blowing fluid via blow pipe, aforementioned insert and/or stretching member. Thus, the pre-form is subjected to radial expansion which follows mechanical stretching and the expansion continues until the entire external surface of the resulting shaped article contacts the entire internal surface of the final blow mold.
A drawback of the just described method is that biaxial orientation of the material in the bottom of a finished article is not as satisfactory as biaxial orientation of the major portion or wall of the article. Therefore, such articles exhibit weakened portions, especially in the transition zone between the bottom and the peripheral wall. Such weakened portions are likely to break, i.e., the improved stability of the peripheral wall as a result of satisfactory biaxial orientation of its material is to no avail because the stability of the remaining portion or portions of the shaped article is much less satisfactory.
Though each of the factors which are responsible for the development of weaker portions in shaped articles produced in accordance with the method disclosed in the aforementioned U.S. Pat. No. 3,781,395 is still now known, it is already established that the development of weakened portions is attributable to several factors. It is also established that two factors exert much greater influence upon the quality (or more accurately stated upon unsatisfactory quality) of shaped articles than all other factors together. The first important factor is that, when a pre-form is expanded into contact with the surfaces surrounding the cavity of the final blow mold, the major portion (peripheral wall) of the finished article undergoes pronounced and desirable biaxial orientation whereas the bottom portion merely expands in the radial direction or expands primarily in the radial direction. This will be readily appreciated since the bottom wall of a bottle or an analogous shaped article is normally located in a plane at right angles to the central vertical axis of the bottle. When the closed end of the pre-form is moved near to or into actual contact with the bottom surface in the final blow mold as a result of mechanical stretching, the admission of blowing fluid entails rather pronounced displacement of the major portion of the pre-form in the axial and radial directions of the mold cavity but the portion including the closed end expands mainly in the radial direction to contact the bottom surface in the final blow mold. In other words, the bottom surface of the final blow mold does not permit any discernible axial expansion in response to admission of compressed blowing fluid into the interior of the pre-form. The second important factor is that, in the majority of instances, mechanical stretching of the pre-form by means of a rod or by grippers results in much more pronounced axial stretching of the major portion of the pre-form, namely of that portion which extends from the open end (neck portion) toward but short of the closed end. In other words, and if the cavity of the final blow mold is upright, with the neck portion of the confined pre-form held between the upper portions of the mold sections, mechanical stretching is much more pronounced in the upper, middle and lower parts than in the lowermost part of the pre-form.